Though most twin pregnancies still result from natural fertilization, an increase in IVF has made them more prevalent over the last several decades. From conception to delivery, there are some significant differences between singleton and twin pregnancies. Mother and babies are at risk for additional complications and require increased monitoring.
Fraternal or dizygotic twins occur when two separate sperm fertilize two separate eggs that implant at the same time. Pregnancies with fraternal twins always have two placentas and two amniotic sacs. Genetically, the children are no more similar looking than siblings born years apart and can be of the same or different sexes.
Identical twins occur when a single sperm fertilizes a single egg. The egg splits, and each half develops into a baby that is genetically identical to the other. Identical twins are always the same sex. Dependingon when the egg splits and implants, each fetus may have its own placenta and amniotic sac, or the two may share one or the other. Both sharing a placenta and having separate amniotic sacs and the opposite pose an increased risk.
When an egg fails to completely separate, the result is conjoined twins. Why separation stops before it is complete is unknown. Conjoined twins can be joined at the chest, base of the spine, length of the spine, pelvis, trunk, head, or head and chest. Pregnancies with conjoined twins are high-risk, and complications depend on where the twins are attached.
A serious complication that can affect twins that share the same placenta is twin-twin transfusion syndrome (TTTS). If the vessels in the placenta are dispensed unevenly, one twin gets more nourishment than the other. The latter twin loses blood volume, which significantly affects development and can be life-threatening. The recipient twin is at risk for increased blood volume, which can lead to life-threatening heart dysfunction from an abnormal increase of amniotic fluid. Sometimes, surgeons can treat this condition in utero with an endoscopic laser ablation procedure, which corrects the imbalance of the blood supply.
Nobody knows what causes identical twins, and every pregnancy has a one in 250 chance of being identical twins. On the other hand, parents are more likely to have fraternal twins if they have a family history of twins on the mother's side or if they used IVF and had multiple embryos implanted. Older mothers are also more likely to have fraternal twins because they have a better chance of releasing multiple eggs during ovulation.
Ultrasound is an important diagnostic tool in twin pregnancies. A difference in crown to rump length between twins with two placentas can indicate chromosomal or congenital abnormalities. Prenatal testing for abnormalities in twin pregnancies is complicated. Screenings are less accurate, and invasive testing has a greater risk of complications. There is also the possibility that one twin has an abnormality, and the other does not, which complicates the pregnancy.
Twin pregnancies have unique nutritional needs. A mother's daily caloric intake should increase by 600 calories a day, twice as much as a singleton pregnancy. Expectant mothers also need additional iron, pre-natal vitamins, and folic acid. Women carrying twins are expected to gain more weight than women pregnant with a single baby, between 37 and 54 pounds for those with a normal pre-pregnancy BMI.
Labor is the same for twin pregnancies as it is for a singleton, but things change significantly once it's time for delivery. In most cases, twin deliveries take place in an operating room. Even if the first baby is born vaginally, it is quite possible the second baby will be breech or require a cesarean section.
Going into labor before 37 weeks is the most common complication of a twin pregnancy. Managing pre-term labor in twins is complex. The doctor may administer medications to accelerate the babies' lung development or reduce the risk of cerebral palsy in infants born between 24 and 32 weeks. Medication can also be given to the mother to stop contractions temporarily.
Mothers who are carrying twins are also at higher risk of developing gestational diabetes. Because the placenta is larger than a singleton pregnancy, it produces more hormones. All pregnant women are screened between 24 and 28 weeks of pregnancy. If they develop gestational diabetes, they must monitor their glucose levels multiple times a day and manage diet and exercise carefully. They require multiple doctor's office visits and may be prescribed medication.
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